Emulsions of oil and water usually are prepared with the add of a surfactant of some sort. The oil and surfactant, if the surfactant is a lipid, are heated to an elevated temperature sufficient to liquify these components, and they are then mixed with the water, also at an elevated temperature, under high agitation and shear conditions, for instance by homogenization, sufficient to form the emulsion. Depending upon the relative concentrations of the several components, either water or oil may be the continuous phase.
Usually, the better cosmetic or lotion emulsions have a high oil content, for instance more than 50% on a weight basis, so that oil is the continuous phase. The emulsions are characteristically smooth, homogeneous, and creamy in texture. In the case of oil-in-water emulsions, where water is the continuous phase, these tend to be more difficult to stabilize against phase separation, and stability often is obtained only with the assistance of a hydrocolloid such as a protein or gum. Protein, for instance, behaves as a film former around the emulsified fat globule, stabilizing the emulsion.
It is known that with certain surfactants, the pH of the emulsion can have a decided effect on the characteristics of the emulsion. This has been observed, for instance, with the use of distilled monoglycerides, wherein an acidic condition seems to give a firmer gel structure having a higher degree of rigidity, more so than one would expect from the proportions of ingredients present.
The present invention is based on the discovery that the properties of an emulsion, where the surfactant is a particular class of polyglycerol esters of fatty acids, can also be carefully controlled by controlling the emulsion pH.
The present invention with regard to the preferred embodiment hereof, particularly is based on the discovery of a critical pH window in which a firm gel can be formed using a lower proportion of lipids than would normally be used to form an emulsion of the same degree of firmness.
A feature of the present invention is that the emulsions are formed and are stable without the use of a hydrocolloid, such as a gum or protein, conventionally employed for stabilization of emulsions.